Conventionally, semipermeable membranes including a cellulose acetate membrane which has an asymmetric structure have been used as water-treatment separation membranes for blocking permeation of dissolved components such as salts (for example, Patent Document 1). However, this membrane has low hydrolysis resistance and microbial resistance, and is further insufficient in fundamental performance as a water-treatment separation membrane, such as salt rejection rate and water permeability. Therefore, the cellulose acetate membrane which has an asymmetric structure has not yet been put to practical use in a wide range of applications, while the membrane has been used for some applications.
In order to correct these defects, a composite semipermeable membrane, which has a microporous supporting membrane with a different material provided thereon to serve as a separation function layer for substantially providing membrane separation performance, has been proposed as a semipermeable membrane which differs from the membrane of the asymmetric structure in terms of configuration. In the case of the composite semipermeable membrane, it is possible to select an optimum material for each of the microporous supporting membrane and the separation function layer, and also a variety of methods can be selected for the membrane forming technique. Most of composite semipermeable membranes which have been commercially available so far have a separation function layer composed of a polyamide, which is obtained by interfacial polycondensation on a porous supporting membrane. Examples of such composite semipermeable membranes include the invention described in Patent Document 2. Furthermore, Patent Document 3 discloses a separation function membrane layer with the structure of a polyamide containing a silicon compound having an alkoxy group.
These composite semipermeable membranes described above provide higher desalination performance, and higher water permeability at the same time, than the cellulose acetate asymmetric membrane. However, such a composite semipermeable membrane using a polyamide has an amide linkage in its main chain, and thus still has insufficient resistance to oxidizing agents, and it is known that the desalination performance and selective separation performance are significantly degraded due to a treatment with chlorine, hydrogen peroxide, etc., for use in sterilization of the membrane.
In that regard, for example, Patent Document 4 and Patent Document 5 disclose a separation function layer obtained by polymerizing an ethylenically unsaturated compound, which has high general versatility for the membrane forming technique and has a wide range of selectivity for raw materials. However, while the composite semipermeable membrane produced with the use of the compound as described above has excellent chemical resistance, either water permeability or separation performance is insufficient.